The present invention relates to capnography in which the concentration of CO.sub.2 in breathing gases is monitored during surgery or for other medical procedures or evaluation. The concentration detection point is advantageously as close to the mouth as possible necessitating a small lightweight unit capable of being placed close to the mouth. The traditional in-line manner of making this measurement is by emitting in the infrared from a heated body, filtering the broadband radiation to the CO.sub.2 absorption band, applying it through a section of the passage from the breathing gases and detecting the change in level of the radiation caused by selective absorption by CO.sub.2. The heated bodies generate much energy in bands where it cannot be used and are subject to problems of aging. The filters are another element in the system that are sources of expense, errors from temperature variation and contamination if not properly sealed. The detectors for the IR range are very temperature dependent and noisy. Mechanical chopping techniques have been used in the past to avoid DC drift errors but are expensive and unreliable.
LED emitters are available in the near IR range, but emitters of appropriate frequency and narrowness in the band of interest have not been available, so that the emitter or source of choice continues to be the heated black or grey body.